On Mon, Jan 04, 2016 at 11:08:19AM +0000, Stefano Stabellini wrote: > On Wed, 30 Dec 2015, Michael S. Tsirkin wrote: > > Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx> > > --- > > Documentation/memory-barriers.txt | 33 ++++++++++++++++++++++++++++----- > > 1 file changed, 28 insertions(+), 5 deletions(-) > > > > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt > > index aef9487..a20f7ef 100644 > > --- a/Documentation/memory-barriers.txt > > +++ b/Documentation/memory-barriers.txt > > @@ -1655,17 +1655,18 @@ macro is a good place to start looking. > > SMP memory barriers are reduced to compiler barriers on uniprocessor compiled > > systems because it is assumed that a CPU will appear to be self-consistent, > > and will order overlapping accesses correctly with respect to itself. > > +However, see the subsection on "Virtual Machine Guests" below. > > > > [!] Note that SMP memory barriers _must_ be used to control the ordering of > > references to shared memory on SMP systems, though the use of locking instead > > is sufficient. > > > > Mandatory barriers should not be used to control SMP effects, since mandatory > > -barriers unnecessarily impose overhead on UP systems. They may, however, be > > -used to control MMIO effects on accesses through relaxed memory I/O windows. > > -These are required even on non-SMP systems as they affect the order in which > > -memory operations appear to a device by prohibiting both the compiler and the > > -CPU from reordering them. > > +barriers impose unnecessary overhead on both SMP and UP systems. They may, > > +however, be used to control MMIO effects on accesses through relaxed memory I/O > > +windows. These barriers are required even on non-SMP systems as they affect > > +the order in which memory operations appear to a device by prohibiting both the > > +compiler and the CPU from reordering them. > > > > > > There are some more advanced barrier functions: > > @@ -2948,6 +2949,28 @@ The Alpha defines the Linux kernel's memory barrier model. > > > > See the subsection on "Cache Coherency" above. > > > > +VIRTUAL MACHINE GUESTS > > +------------------- > > + > > +Guests running within virtual machines might be affected by > > +SMP effects even if the guest itself is compiled within > > ^ without Right - this is fixed in v2. Could you review that one please? > > +SMP support. > > + > > +This is an artifact of interfacing with an SMP host while > > +running an UP kernel. > > + > > +Using mandatory barriers for this use-case would be possible > > +but is often suboptimal. > > + > > +To handle this case optimally, low-level __smp_mb() etc macros are available. > > +These have the same effect as smp_mb() etc when SMP is enabled, but generate > > +identical code for SMP and non-SMP systems. For example, virtual machine guests > > +should use __smp_mb() rather than smp_mb() when synchronizing against a > > +(possibly SMP) host. > > + > > +These are equivalent to smp_mb() etc counterparts in all other respects, > > +in particular, they do not control MMIO effects: to control > > +MMIO effects, use mandatory barriers. > > > > ============ > > EXAMPLE USES > > -- > > MST > > -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
